Semiconductor devices have been known which are fabricated by soldering metal leads and conductive patterns on a wiring board together, and then encapsulating the leads (inner leads), the mounting surface of the wiring board, and the electronic components mounted on the wiring board with a mold resin. To increase heat dissipation, some of these semiconductor devices are provided with a heatsink on the exposed surface uncovered with the mold resin, which is the opposite side of the wiring board from the mounting surface.
Such a semiconductor device, however, has a larger package size than a discrete device, such as a switching device. If the package warps due to a temperature change, the contact area between the wiring board and the heatsink decreases, resulting in lower heat dissipation. The degree of the package warping depends on the difference between the coefficients of linear expansion of the mold resin and the wiring board. In view of this fact, a mold resin with a low coefficient of linear expansion is used to achieve a small difference between these coefficients of linear expansion and thus a low degree of the package warping due to a temperature change.
It should be noted that Patent Literature 1 discloses providing a lead with a projection, which has a flat circular top surface, by dowel out pressing, thereby improving the reliability of a soldering joint between a lead and another lead (inner lead) in a semiconductor device.